Most of presently available welding robots are of a teaching play-back type. These known robots have required that several operation programs be taught to a single robot in order to cope with multi-kind and small quantity production. Further, it has been desired to provide measures for reducing the manhours of effective teaching of an operation program to an welding robot in view of a lack of skilled workers for teaching a robot.
A teaching method which is widely used, in general, at the present time, requires that a teaching worker use manipulating switches in a manipulation box for designating a motion of a robot so as to cause the robot to take a predetermined motion while the teaching worker monitors the motion of the robot in order to teach an operation program. This method presents problems such as requiring a complex teaching operation for a workpiece to be welded which requires a curved welding line, direction change or the like, and further requires repetitions trial operations in order to obtain a desired motion for the robot. Accordingly, it takes a long time for the teaching of the robot.
In view of the above, various teaching measures that don't use a manipulation box as mentioned above, have been proposed, and have been practically used.
Of the above-mentioned measures, one is the so-called off-line teaching method for teaching an absolute position on the coordinates set on a working field including a robot. For example, there have been proposed a method of teaching an absolute position with the use of computer simulation, a method in which a position teaching unit incorporating several light emitting elements, instead of a torch, is moved along a welding line on a workpiece to be welded, and light emitted therefrom is detected by image sensors fixed to a working field so as to teach a robot, as disclosed in Japanese Laid-Open Patent No. 60-136806 or Japanese Patent Publication No. 64-4875. In these methods, a high degree of accuracy is reburied for positioning a robot. Further, the confirmation of the presence of any interference between a robot arm, a welding torch or a welding cable and a workpiece to be welded, a peripheral jig or the like is required.
Further, there are other methods in which teaching is made while a robot is actually operated. A first one of these methods is a direct teaching method in which a robot sometimes detects and stores in memory its motion while a worker who grips a welding torch or a grip part which a dummy of the torch fixed to the distal end part of a robot arm, leads the torch or the like so as to follow a welding line. For example, Japanese Laid-Open Patent No. 56-85106 discloses a method of detecting the motion, in which a force detector for detecting a direction in which the worker leads the grip part and a force for leading the grip part is provided, and the position and posture of the tip end of the torch is computed from an output signal from the detector. Since the direct teaching method allows the worker to manipulate the robot with his institution without being aware of the coordinates set on a robot, it is easily accomplished by the teaching worker. However, the installed force detector may interfere with a workpiece or a jig so that it cannot be manipulated. Further, certain shapes of a workpiece forces the worker to take an unreasonable posture. Further, there is a possible risk such that the worker may accidentally touch the robot and be harmed.
A second method allows a robot to automatically recognize a welding start point and a weld line with the use of a sensor so that the robot teaches while it autonomously follows the welding line. This method can eliminate the necessity of confirmation of the interference and of correction to a teaching point, and eliminate the necessity of teaching which is performed by a worker in close proximity with a robot. Further, since the robot automatically performs positioning and sets a posture with the use of data from the sensor, it is possible to aim at uniforming the quality of teaching without being dependent upon a worker's skill.
Means for automatically recognizing a welding start point and a welding line, is classified mainly into two types. The first type uses a distance sensor or an image sensor utilizing a laser beam, ultrasonic waves or the like, and the second type uses the welding work itself including a welding wire, a welding arc or the like.
In general, most of the means for automatically recognizing a welding start point and a welding line with the use of the distance sensor or the image sensor utilizing a laser beam, ultrasonic waves or the like require additional attachment of the sensor to a robot in the vicinity of a welding torch, and accordingly, the interference between the sensor and a workpiece, a jig or the like may be a hindrance, since a small sized accurate sensor can not be obtained at the present time, and further, affections caused by surface conditions of a workpiece, enviromantal illumination, an ambient temperature or the like are not negligible. Thus, there has been offered such a problem that a workpiece, a jig or a working environment which are applicable are subjected to great restrictions.
As disclosed in Japanese Laid-Open Patent No. 54-15441, a touch sensor using a welding wire as a wire earth is adapted to recognize a position on a workpiece from the position of a robot arm at a time when the welding wire as one of opposite electrodes applied with a voltage and the surface of the workpiece as another one of opposite electrodes make contact with each other so as to obtain an electrical communication during movement of the welding wire by a robot, and with the repetitions of the above-mentioned procedure at several points, the welding line can be detected. However, in this procedure, the detecting action is not so fast since sensing is required at several positions for every teaching point, and accordingly, as the workpiece becomes more complicated a number of teaching points increases, thus requiring longer teaching time. Then presents such a problem that the practical usability thereof is remarkably deteriorated.
Further, as disclosed in Japanese Laid-Open Utility Model No. 54-55635, an arc sensor utilizing a welding arc is of a type that recognizes the position of a welding line with the use of variations in a welding current signal as a data source, that is, variations in welding current caused by variations in the distance between a welding tip and a mother material, which are in turn caused by weaving a welding torch in crossing with the welding line within the bevel of a welding joint, so as to recognize the position of the welding line. However, this method cannot be repeated since the arc itself has to serve as a sensor, so that the teaching must be made during an actual welding operation. Further, there have been offered problems such as the trace of a welding line at a high speed is difficult, practical application to a lap joint of thin plates is inappropriate, the performance of the trace is greatly affected by welding terms, the control of the posture of a torch is difficult and so forth.
The present invention is devised in order to solve the above-mentioned problems, and accordingly, one object of the present invention is to provide a sensor having a size and a shape which do not reduce the accessibility to a workpiece, that is applicable to various workpiece shapes and various working environments, and that is capable of automatically recognizing the position of a welding line and the posture of a torch with respect to a workpiece at a high speed without carrying out a practical welding operation, and to provide a welding robot using the sensor, which can easily carry out a safe and effective teaching method with reduced man hours.